Gray iron is a group of ferrous alloys that contain a relatively large percentage of carbon in the form of flake graphite. Gray iron generally contains more than 95% iron element, while the main alloying elements are carbon and silicon. The amount of carbon in gray iron typically is in the range of 2.1%-4%. Gray iron is relatively easy and inexpensive to make. Compared to the more modern engineered irons, gray iron has a lower tensile strength and lower ductility. In other words, it may fail more easily, and its mode of failure may be by sudden fracture. Gray iron is used for engine components where tensile strength is not critical, for example, engine blocks, engine cylinder heads, engine liners, pump housings, and valve bodies. There are several advantages to using gray iron to make certain engine components. For example, gray iron transfers heat more quickly and easily than steel. Also, gray iron has noise damping characters that result in lower engine noise.
It is expected that a stricter environment regulation be enforced in the future, which demands lower exhaust emissions from internal combustion engines. One way to achieve this is to increase combustion temperatures and pressures in internal combustion engines. This may require that the engine components made from gray iron have increased strength and hardness. As a result, there is a desire for a new method to improve the tensile strength and hardness of engine components made from gray iron.
It has been known that mechanical properties of a material are a function of the material's microstructure, and that such properties may be controlled by chemistry and physical processing. For example, one method of enhancing the tensile strength of steel is described in U.S. Pat. No. 5,885,370 (the '370 patent) issued to Shimotomai et al. The '370 patent describes a method of providing magnetic heat treatment to steel to refine the microstructure and improve the mechanical properties of the steel.
While the method of the '370 patent may be effective for improving mechanical properties of steel, the method of the '370 patent includes several disadvantages. For example, according to the disclosure of the '370 patent, the method may only be effective on steel with a carbon percentage limited to a range of 0.01% to 2% by mass. Thus, the disclosed method may not be applicable to gray iron that typically has a carbon percentage in a range of 2.1% to 4% by mass. In addition, the method disclosed in the '370 patent requires applying a magnetic field having a gradient limited to a particular range. Maintaining a magnetic field within this particular range can be complicated and/or expensive.
The disclosed method is directed to overcoming one or more of the problems set forth above.